Observations of a Strong Unidentified Microwave Line and of Emission from the OH Molecule

Weaver, Harold F.; Williams, David R.; Dieter, Nannielou H.; Lum, W. T.

doi:10.1038/208029a0

Cited by 180 publications

(78 citation statements)

References 3 publications

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“…In optically thin regions under conditions of local thermodynamic equilibrium (LTE), the expected intensity ratios are approximately 1:5:9:1 for the 1612, 1665, 1667, and 1720MHz lines, respectively (Elitzur 1992. However, in several observations different line ratios were measured (McGee et al 1965), and in some cases the intensity of a given line significantly exceeded that predicted using LTE (Weaver et al 1965). The strong anomalous line intensities were explained by postulating maser action for the corresponding transitions.…”

Section: Introductionmentioning

confidence: 99%

DICKE’S SUPERRADIANCE IN ASTROPHYSICS. II. THE OH 1612 MHz LINE

Rajabi

Houde

2016

ApJ

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We apply the concept of superradiance that was introduced by Dicke in 1954 to the OH molecule 1612 MHz spectral line, which is often used for the detection of masers in the circumstellar envelopes of evolved stars. Because the detection of 1612 MHz OH masers in the outer shells of envelopes of these stars implies the existence of a population inversion and a high level of velocity coherence, and that these are two necessary requirements for superradiance, we investigate whether superradiance can also happen in these regions. Superradiance is characterized by high-intensity, spatially compact, burst-like features taking place over timescales on the order of seconds to years, depending on the size and physical conditions present in the regions harboring such sources of radiation. Our analysis suggests that superradiance provides a valid explanation for previous observations of intensity flares detected in that spectral line for the U Orionis Mira star and the IRAS 18276-1431 preplanetary nebula.

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Section: Introductionmentioning

confidence: 99%

DICKE’S SUPERRADIANCE IN ASTROPHYSICS. II. THE OH 1612 MHz LINE

Rajabi

Houde

2016

ApJ

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“…Intense maser action in cosmic molecular clouds was discovered in 1965 (Weaver et al 1965) from observations of OH, and later from H 2 O, SiO, and CH 3 OH. In the case of water vapor, the commonly observed masers emit in the 6 16 -5 23 transition at 22235 MHz (1.35 cm wavelength).…”

Section: Cosmic Masersmentioning

confidence: 99%

Observational evidence for massive black holes in the centers of active galaxies

1999

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Naturally occurring water vapor maser emission at 1.35 cm wavelength provides an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Because of the exceptionally fine angular resolution, 200 microarcseconds, obtainable with very long baseline interferometry, accompanied by high spectral resolution, < 0.1 km s −1 , the dynamics and structures of these disks can be probed with exceptional clarity. The data on the galaxy NGC 4258 are discussed here in detail. The mass of the black hole binding the accretion disk is 3.9 × 10 7 M ⊙ . Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly measured with VLBI over a period of a few years. These measurements also allow the distance from the earth to the black hole to be estimated to an accuracy of 4 percent. The status of the search for other maser/black hole candidates is also discussed.

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“…Subsequent studies found absorption toward all four of the "18 cm lines" first toward the Galactic center (Gardner et al 1964) and, later, many galactic lines of sight (e.g., Goss 1968). Soon, also emission near 1665 MHz was discovered toward prominent HII regions, i.e., sites of recent star formation (Weaver et al 1965). This and the detection of weaker emission at the frequencies of Appendices are available in electronic form at http://www.aanda.org the F = 2−2 and 1−2 OH hfs lines (at 1667 and 1720 MHz) allowed Weinreb et al (1965) to identify OH as the carrier of this peculiar emission.…”

Section: Introductionmentioning

confidence: 99%

SOFIA observations of far-infrared hydroxyl emission toward classical ultracompact HII/OH maser regions

Csengeri

Menten

Wyrowski

et al. 2012

A&A

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Context. The hydroxyl radical (OH) is found in various environments within the interstellar medium (ISM) of the Milky Way and external galaxies, mostly either in diffuse interstellar clouds or in the warm, dense environments of newly formed low-mass and high-mass stars, i.e., in the dense shells of compact and ultracompact HII regions (UCHIIRs). Until today, most studies of interstellar OH involved the molecule's radio wavelength hyperfine structure (hfs) transitions. These lines are generally not in local thermodynamical equilibrium (LTE) and either masing or over-cooling complicates their interpretation. In the past, observations of transitions between different rotational levels of OH, which are at far-infrared (FIR) wavelengths, have suffered from limited spectral and angular resolution. Since these lines have critical densities many orders of magnitude higher than the radio wavelength ground state hfs lines and are emitted from levels with more than 100 K above the ground state, when observed in emission, they probe very dense and warm material. Aims. We aim to probe the warm and dense molecular material surrounding the UCHIIR/OH maser sources W3(OH), G10.62−0.39 and NGC 7538 IRS1 by studying the 2 Π 1/2 , J = 3/2−1/2 rotational transition of OH in emission and, toward the last source also the molecule's 2 Π 3/2 , J = 5/2−3/2 ground-state transition in absorption. Methods. We used the Stratospheric Observatory For Infrared Astronomy (SOFIA) to observe these OH lines, which are near 1.84 THz (163 μm) and 2.51 THz (119.3 μm), with high angular (∼16 /11 ) and spectral resolution (better than 1 km s −1 ). Results. We clearly detect the OH lines, some of which are blended with each other. Employing non-LTE radiative transfer calculations we predict line intensities using models of a low OH abundance envelope versus a compact, high-abundance source corresponding to the origin of the radio OH lines. From the observed velocities and line-widths we can place constraints on the origin of the emission, and with detailed modeling we show for instance that the OH emission of W3(OH) comes from the UCHIIR and not from the envelope of the nearby hot-core. Conclusions. The FIR lines of the OH molecule provide important information on the density and temperature structure of UCHIIRs. Taking a low-abundance envelope component is not sufficient to reproduce the spectra for W3(OH) and G10.62−0.39, a compact, high OH column density source -corresponding to the OH radio emitting sources is definitively required.

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Observations of a Strong Unidentified Microwave Line and of Emission from the OH Molecule

Cited by 180 publications

References 3 publications

DICKE’S SUPERRADIANCE IN ASTROPHYSICS. II. THE OH 1612 MHz LINE

DICKE’S SUPERRADIANCE IN ASTROPHYSICS. II. THE OH 1612 MHz LINE

Observational evidence for massive black holes in the centers of active galaxies

SOFIA observations of far-infrared hydroxyl emission toward classical ultracompact HII/OH maser regions

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